actually i used folder lock to lock some files.It was only a trail version and expired after some days. Then i couldn't open the locked files.So i uninstalled folder lock. All the files i locked had NUL.prefix . Any other solution? coz i tried the \\?\ prefixing it only gave me a access denied message

The fact you're getting "access denied" instead of "could not find" means you entered the right string for the file, but something else is stopping you. Maybe the file has the read-only attribute. Try "del /f" which overrides that. If a file had hidden, system and read-only attributes, you could use "del /a /f". If you still get access denied, there may be a permissions issue, and you should try again from a command prompt running as Administrator.